1. Field of the Invention
The present invention relates to an air-fuel-ratio control apparatus for an internal combustion engine, which apparatus is applied to an internal combustion engine provided with an upstream air-fuel-ratio sensor disposed in an exhaust passage to be located upstream of a catalyst unit disposed in the exhaust passage, and feedback-controls the air-fuel ratio (hereinafter referred to as “air-fuel ratio”) of the gas mixture supplied to the internal combustion engine on the basis of the output of the upstream air-fuel-ratio sensor.
2. Description of the Related Art
For example, Japanese Patent Application Laid-Open (kokai) No. 2004-183585 discloses a conventional air-fuel-ratio control apparatus of such a type. In the disclosed air-fuel-ratio control apparatus for an internal combustion engine (hereinafter sometimes simply referred to as “engine”), a target air-fuel ratio is determined on the basis of the operation state of the engine. An upstream-side feedback correction value is calculated on the basis of the value corresponding to the deviation of the air-fuel ratio (detected air-fuel ratio), which corresponds to the output value from the upstream air-fuel ratio sensor, from the target air-fuel ratio (specifically, the deviation of the value (detected cylinder fuel supply quantity), which is obtained by dividing a cylinder intake air quantity by the detected air-fuel ratio, from the value (target cylinder fuel supply quantity), which is obtained by dividing the cylinder intake air quantity by the target air-fuel ratio). A fuel injection quantity is calculated on the basis of the upstream-side feedback correction value and a base fuel injection quantity, which is a quantity of fuel for obtaining the target air-fuel ratio, and the instruction for injecting the fuel in the fuel injection quantity is given to an injector, whereby the air-fuel ratio is feedback-controlled.
Meanwhile, when the target air-fuel ratio changes, the fuel injection quantity (accordingly, air-fuel ratio) changes due to the change of the base fuel injection quantity. In general, it takes a predetermined time (hereinafter referred to as “dead time”) for the exhaust gas generated upon the combustion of the fuel to reach the upstream air-fuel-ratio sensor from the time when the instruction for injecting fuel. Accordingly, the change in the air-fuel ratio appears as the change in the detected air-fuel ratio with the delay of the dead time. Thus, when the target air-fuel ratio changes, the detected air-fuel ratio (accordingly, detected cylinder fuel supply quantity) changes with the delay of the dead time.
On the other hand, when the target air-fuel ratio changes, the target cylinder fuel supply quantity immediately changes. Therefore, the timing of the change in the target cylinder fuel supply quantity does not coincide with the timing of the change in the detected cylinder fuel supply quantity. Accordingly, when the deviation of the detected cylinder fuel supply quantity from the target cylinder fuel supply quantity itself (current value) is used as the aforesaid deviation, the deviation (accordingly, the upstream-side feedback correction value) temporarily increases, whereby there may be the case in which relatively great fluctuation is produced in the air-fuel ratio. This is unpreferable for promptly converging the air-fuel ratio to the target air-fuel ratio.
In view of this, in the disclosed apparatus, the target cylinder fuel supply quantity at the point the dead time before the present point in time is used, instead of the target cylinder fuel supply quantity itself, in order that the timing of the change in the target cylinder fuel supply quantity coincides with the timing of the change in the detected cylinder fuel supply quantity, upon calculating the aforesaid deviation (accordingly, the upstream-side feedback correction value).
The air-fuel-ratio control apparatus disclosed in the aforesaid application entails, however, the problem described below. The case in which the target air-fuel ratio sharply changes (e.g., the case in which the target air-fuel ratio changes in a stepwise manner) is now considered. In this case, the target cylinder fuel supply quantity sharply changes the dead time after the point when the target air-fuel ratio sharply changes. On the other hand, since the upstream air-fuel-ratio sensor has a response delay, the detected cylinder fuel supply quantity relatively gently changes with the response delay the dead time after the point when the target air-fuel ratio sharply changes.
Specifically, although the timing of the change in the target cylinder fuel supply quantity and the timing of the change in the detected cylinder fuel supply quantity coincides with each other, the degree of the delay of the respective changes greatly differ from each other after the timing of the change. Therefore, the upstream-side feedback correction value might still temporarily increase, resulting in entailing a problem that it is difficult to promptly converge the air-fuel ratio to the target air-fuel ratio.